1. Field of the Invention
The present invention relates to an imaging lens unit. Particularly, the present invention relates to an imaging lens unit with a dual-lens configuration for capturing high-resolution images.
2. Description of Related Art
As image pickup devices demonstrate higher performance with a decrease in pixel size or the like, image pickup apparatus have smaller size and higher image quality. This creates a demand for smaller imaging lens units. For the achievement of higher image quality and smaller size, a triple-lens configuration has been used. The use of the triple-lens configuration enables the capture of high-quality images such as SXGA (1.3 megapixels) and UXGA (2.0 megapixels).
On the other hand, a dual-lens configuration is sometimes used for the purpose of size and cost reduction. However, dual-lens configurations of related arts are not suitable for capturing high-quality images, and they are only compatible with low-resolution imaging devices such as VGA (0.3 megapixels). Therefore, there is an increasing demand for development of megapixel dual lenses which are capable of capturing high-quality images.
The capture of higher-quality images can be achieved if ray is incident on an image pickup device at a more vertical angle. If ray is incident on an image pickup device more vertically, the ray can be effectively introduced to a photoelectric conversion device of the image pickup device, thereby creating better quality of images. One known approach is to place a microlens array on the front side of an image pickup device in order to reduce adverse effects caused by an oblique incidence to the image pickup device. Even with such an approach, however, a large angle of incident to an image pickup device hinders the creation of high-quality images. Thus, the development of a lens unit which approximates an angle of incident onto an image pickup device to a vertical angle is desired. The angle of incident refers to an angle between a normal to an incident surface and an incident ray. An angle of refraction, which is described later, refers to an angle between a normal to an incident surface and a refracted ray. If an incident surface is curved, a normal to a tangent plane at the point of incidence of ray is referred to as a normal to an incident surface.
The structure of using two meniscus lenses for an imaging lens as a dual-lens configuration with small aberration is proposed in Japanese Unexamined Patent Application Publication No. 2003-232990, for example. According to this technique, a first lens has a positive refractive power with a convex surface toward the object side, and a second lens has a positive refractive power with a convex surface toward the image side. Each lens has an aspherical surface on which aspherical correction has been made.
In order to reduce the size of image pickup apparatus, it is necessary to shorten an optical length in an imaging lens unit which is used in an image pickup apparatus. The optical length is a distance on an optical axis from the apex of a lens surface of the first lens on the object side to an image pickup surface. However, the optical length is long when ray from an object is incident on an image pickup device at a substantially vertical angle because the ray is applied to the image pickup surface substantially vertically. It has therefore been extremely difficult to both shorten the optical length and approximate the angle of incident of ray on an image pickup surface to a vertical angle.
The present invention has been accomplished to solve the above problems and an object of the present invention is thus to provide an imaging lens unit with a dual-lens configuration which has a short optical length, approximates an angle of incident of ray on an image pickup surface to a vertical angle, and corrects aberration to be small enough for the integration density of an image pickup device.